When a load which is driven by a power switching element is short-circuited, an excessively large current rapidly flows through the power switching element. Various circuits for protecting the power switching element from the overcurrent have been proposed. However, in a drive circuit provided with such a protection circuit, the protection circuit erroneously operates, for example, due to a switching noise of the power switching element, which disadvantageously results in an unintended stop of the drive of the power switching element.
On the other hand, a protection circuit described in Patent Literature 1 reduces a gate voltage of a power switching element by resistance voltage division to reduce a current when a short circuit of a load is detected. Further, when a current higher than a short-circuit detection threshold continuously flows for a predetermined time, the protection circuits determines a short circuit of the load and stops the drive of the power switching element.
However, in the protection circuit described in Patent Literature 1, the gate voltage is uniformly reduced in the protection against a short circuit. Thus, variations in the threshold voltage caused by manufacture variations in the power switching element produce variations in a current flowing through the power switching element. When a current value falls below the short-circuit detection threshold within the above predetermined time due to the variations in the current value, the short-circuit protection operation is erroneously released. Thus, it is necessary to set a large short-circuit detection threshold to prevent the erroneous release. As a result, the stress on the power switching element increases.
Further, the waveform of the gate voltage is rounded by the resistance for voltage division and the gate capacitance of the power switching element, and the time constant during a gate voltage reduction increases. Thus, a time required for reducing the current of the power switching element increases, which results in a large stress.
On the other hand, a power converter described in Patent Literature 2 employs a method that reduces a gate voltage of a power switching element by a Zener diode when a short circuit is detected.
The method described in Patent Literature 2 can rapidly reduce the gate voltage and thus can solve the problem of the time constant during the gate voltage reduction. However, a Zener diode typically has a large characteristic variation caused by temperature or manufacturing variations. Thus, it is not possible to solve the problem of the erroneous release of the short-circuit protection operation.
Further, it is difficult to control a drop speed of the gate voltage during the gate voltage reduction. Thus, ringing is generated in the current of the power switching element, which results in application of a surge voltage to the power switching element.